European Patent Application No. EP1457400_A1 describes, inter alia, a pre-filling device configured to prepare braking in a braking system by supplying the brakes, through the master cylinder, prior to the actual braking, a pre-filling pressure having a predetermined value, usually a medium/low pressure value.
Conventionally, the value of the pre-filling pressure is a function of the hydro-boost pressure used by the hydro-booster, for example about 20-25 bar, and a delta pressure, for example 19 bar, obtained by a decrease in pressure from the hydro-boost pressure, the decrease being generated by a spring element which operates between a pair of balls provided in the pre-filling device.
For example, a variation of the pressure used by the hydro-booster will give:20−19=1, or  a)25−19=6,  b)therefore a variation between 1 and 6 bar, while not considering any small tolerances of the delta pressure.
While appreciating that the conventional pre-filling device is, in general, very effective, it has been observed, in the course of experimental tests, that during use the hydro-boost pressure is not a constant value but, in particular in agricultural vehicles, it is a value that can be comprised, for example, between 20 and 25 Bar.
The variations observed, in some cases, assuming a constant counter-pressure generated by the spring element and acting on the pair of valves of the pre-filling device, prevent to ensure that the pre-filling pressure is maintained within very narrow margins, comprised, for example, between 3-4 Bar.
This fact implies, of course, that the braking of the vehicle can present different behaviors upon varying of the hydro-boost pressure and thus of the pre-filling pressure.
Hydro-boost devices (hydro-boosters) comprising mechanical valves are also available. For instance:
U.S. Pat. No. 4,337,686 A describes a hydro-booster comprising an input rod, a sliding piston, placed around the input rod and adapted to separate an oil inlet chamber from an oil outlet chamber, and a valve able to allow a predetermined minimum passage of oil between the two chambers;
European Patent Application No. EP2412593A1 describes a hydro-booster comprising a hydro-boost chamber, an input rod, a hydro-boost passageway between a reservoir and the hydro-boost chamber, and a gasket adapted to selectively isolate the hydro-boost passageway from the hydro-boost chamber depending on the position of the input rod;
European Patent Application Nos. EP0290785A1 and EP0290786A1 describe a hydro-booster comprising a valve which is arranged internally to a piston and is opened by actuating the brake pedal so that a pressurized fluid coming from a reservoir can act on the piston;
U.S. Pat. No. 4,179,980 A describes a hydro-booster comprising a primary piston, a control rod, a charaber located between the primary piston and the control rod and a control valve, actuated by the control rod, for controlling the fluid pressure in the chamber;
German Patent Application No. DE1576087A1 describes a hydro-booster comprising a piston, a control rod, communication ducts between a fluid inlet and the piston, and a valve for controlling a fluid flow through the communication ducts.
In summary, the related art does not provide teachings to keep the pressure of the pre-filling within very narrow margins, upon varying of the hydro-boost pressure, so as to ensure a high degree of uniformity in the pre-filling braking of the vehicle.